delegate_matcher.rb

# RSpec matcher to spec delegations.
# Forked from https://gist.github.com/txus/807456 with added bug fixes
#
# Usage:
#
#     describe Post do
#       it { should delegate(:name).to(:author).with_prefix } # post.author_name
#       it { should delegate(:month).to(:created_at) }
#       it { should delegate(:year).to(:created_at) }
#       it { should delegate(:something).to(:'@instance_var') }
#     end

RSpec::Matchers.define :delegate do |method|
  match do |delegator|
    @method = @prefix ? :"#{@to}_#{method}" : method
    @delegator = delegator

    if @to.to_s[0] == '@'
      # Delegation to an instance variable
      old_value = @delegator.instance_variable_get(@to)
      begin
        @delegator.instance_variable_set(@to, receiver_double(method))
        @delegator.send(@method) == :called
      ensure
        @delegator.instance_variable_set(@to, old_value)
      end
    elsif @delegator.respond_to?(@to, true)
      unless @delegator.method(@to).arity == 0
        raise "#{@delegator}'s' #{@to} method does not have zero arity (it expects parameters)"
      end
      @delegator.stub(@to).and_return receiver_double(method)
      @delegator.send(@method) == :called
    else
      raise "#{@delegator} does not respond to #{@to}"
    end
  end

  description do
    "delegate :#{@method} to its #{@to}#{@prefix ? ' with prefix' : ''}"
  end

  failure_message_for_should do |text|
    "expected #{@delegator} to delegate :#{@method} to its #{@to}#{@prefix ? ' with prefix' : ''}"
  end

  failure_message_for_should_not do |text|
    "expected #{@delegator} not to delegate :#{@method} to its #{@to}#{@prefix ? ' with prefix' : ''}"
  end

  chain(:to) { |receiver| @to = receiver }
  chain(:with_prefix) { @prefix = true }

  def receiver_double(method)
    double('receiver').tap do |receiver|
      receiver.stub(method).and_return :called
    end
  end
end

I think that you should check if the method has an arity of -1 or 0 since either are valid.

Hi Simeon,

I've forked your gist at https://gist.github.com/awinograd/6158961.

It fixes the arity preoblem seanselltl metioned (which I also experienced) as well as adds support for a custom prefix for the delegation that can be added to the model's delegation.

delegate :name, to: :author, prefix: :any
it { should delegate(:name).to(:author).with_prefix(:any) } # post.any_name

Hello Simeon,
Do you have a license that you would like to release this code under?
I spoke with Alec Winograd about his fork, he's added an MIT License to his modifications.
Thank you, hope all is well.

I am happy to offer this code to the community under the MIT License.

Awesome and thank you very much.